Lecture 33

Lecture 33 - ECE 352: Lecture 33 Semiconductor Lasers * The...

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Unformatted text preview: ECE 352: Lecture 33 Semiconductor Lasers * The laser requirements * Transitions between bands Momentum conservation Population inversion * Confinement of carriers: heterojunctions * Laser operation Mirrors and optical confinement Distributed reflectors--VCSELS Nick Holonyak Inventor of LED Semiconductor devices may also be exploited to realize laser systems * This requires the use of DIRECT GAP SEMICONDUCTORS In which optical transitions are FAVORABLE * Recall that Si is NOT a direct semiconductor while GaAs IS Semiconductor Diode Laser IN A DIRECT GAP SEMICONDUCTOR THE VALENCE BAND MAXIMUM AND THE CONDUCTION BAND MINIMUM OCCUR AT THE SAME WAVEVECTOR LIGHT E ( k ) k Robert N. Hall Inventor of semiconductcor laser Laser action may occur in HEAVILY DOPED p-n diodes * In which the intrinsic BAND BENDING is VERY STRONG * When the diode is forward biased a very LARGE current then flows And electron-hole RECOMBINATION causes laser action Semiconductor Diode Laser WITH STRONG FORWARD BIAS WE HAVE n p > p p (the minority population is larger than the majority population) AND SO HAVE ACHIEVED A POPULATION INVERSION. ELECTRONS THEN RECOMBINE WITH HOLES AND EMIT PHOTONS IN THE SAME PROCESS. UNBIASED p-n JUNCTION AT EQUILIBRIUM SAME JUNCTION WITH STRONG FORWARD BIAS P-TYPE N-TYPE P-TYPE N-TYPE ACTIVE REGION The initial recombination is a SPONTANEOUS process but is reinforced * By MULTIPLE reflection of photons from the ends of the diode * Eventually a THRESHOLD VOLTAGE is reached Beyond which STIMULATED laser action occurs Semiconductor Diode Laser HEAVILY N-DOPED HEAVILY P-DOPED ACTIVE REGION LIGHT INTENSITY APPLIED VOLTAGE THRESHOLD SPONTANEOUS EMISSION STIMULATED EMISSION A typical laser emits light at a CENTRAL wavelength * Surrounded by a number of other wavelength components Semiconductor Diode Laser BELOW THE THRESHOLD VOLTAGE ABOVE THE THRESHOLD VOLTAGE EMISSION SPECTRA FROM A GALLIUM ARSENIDE LASER NEAR THRESHOLD 8600 8500 8400 WAVELENGTH (A) 8600 8500 8400 WAVELENGTH (A) The Semiconductor Laser WITH STRONG FORWARD BIAS WE HAVE n p > p p (the minority population is larger than the majority population) AND SO HAVE ACHIEVED A POPULATION INVERSION. ELECTRONS THEN RECOMBINE WITH HOLES AND EMIT PHOTONS IN THE SAME PROCESS. UNBIASED p-n JUNCTION AT EQUILIBRIUM SAME JUNCTION WITH STRONG FORWARD BIAS P-TYPE N-TYPE P-TYPE N-TYPE ACTIVE REGION The Semiconductor Laser When we inject electrons into the p-type region, the density is increased, and we can make the statement that Similarly, when we inject holes into the n-type region, the density is increased, and we can say that The Semiconductor Laser WHAT DOES THIS MEAN??????...
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This note was uploaded on 10/28/2009 for the course EEE 352 taught by Professor Ferry during the Fall '08 term at ASU.

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Lecture 33 - ECE 352: Lecture 33 Semiconductor Lasers * The...

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